A member of the Kremlin's human rights council says there are reasons to believe the prime suspect in the killing of Russian opposition leader Boris Nemtsov confessed under torture.

Andrei Babushkin told reporters Wednesday that he had seen "numerous wounds" on the body of Zaur Dadayev during a visit to the suspect's prison cell on Tuesday.

Babushkin wrote on his website that there were "reasonable grounds to believe" that Dadayev and another suspect were tortured while in custody. Babushkin also told the French news agency that he and other members of the council who visited the prison believe Dadayev "confessed under torture."

Meanwhile, a Moscow newspaper reported Wednesday that Dadayev has reversed his confession. Moskovsky Komsomolets quoted the suspect as saying he confessed to killing Nemtsov because authorities promised to release a former colleague who was also detained.

Dadayev, who was deputy commander of a special Chechen Interior Ministry battalion, said he planned to plead innocent when he was brought before a judge. But he said once he was brought to court, he was not given a chance to speak.

On Sunday, a Russian judge said Dadayev had signed a confession, although no details were given about his alleged role in the murder.

A total of five suspects were detained and questioned about the February 27 shooting death of Nemtsov, a staunch foe of Russian President Vladimir Putin. Nemtsov was shot four times in the back as he walked across a bridge with his girlfriend in sight of the Kremlin.

The judge said a second suspect charged with murder in Nemtsov's death, Anzor Gubashev, had denied his involvement.

Television footage showed Gubashev, who worked for a private security firm in Moscow, sitting in a jail cell holding a piece of paper over his face to hide it. Other images showed heavily armed police quickly escorting three hunched-over suspects into a small courtroom and locking them in barred cages.

Besides Dadayev and Gubashev, authorities identified the other three suspects as Gubashev's younger brother Shagid, Ramsat Bakhayev and Tamerlan Eskerkhanov.

Authorities said a sixth suspect blew himself up with a grenade as police tried to detain him in the Chechen capital, Grozny.

Before the judge disclosed Dadayev's alleged involvement in the killing, his mother, Aaimani Dadayeva, told the Interfax news agency, "I can't believe it. He could not have committed this crime." She said her son has worked for Chechen law enforcement agencies for the last decade.

Despite the detention of the five suspects, no information has emerged about a possible motive for the killing. Nemtsov's allies say his assassination was ordered at the highest levels of the Russian government to silence critics.

All five suspects are from the restive North Caucasus region, where Russia has fought two fierce wars in the last 20 years against separatists in Chechnya. Low-level violence continues in the region, where security forces continue to clash with members of rebel groups that are part of a regional Islamist insurgency.

Putin has called Nemtsov's killing "a provocation" and vowed the government would do everything to ensure those responsible are "properly punished."

The murder occurred two days before Nemtsov was to have led an anti-war and anti-Putin rally in Moscow. The opposition figure had also been working on a report about Russian military involvement in the Ukraine conflict.